Subunit stoichiometry and juxtaposition of the photosynthetic coupling factor 1: Immunoelectron microscopy using monoclonal antibodies.

Monoclonal antibodies specific to the alpha subunits of the photosynthetic coupling factor 1 (CF(1)) were used as marker molecules in an electron microscopic analysis of the subunit organization of this enzyme. Immune complexes were obtained by incubation of CF(1) with saturating amounts of anti-alpha-subunit IgG, isolated by gel filtration, and visualized by electron microscopy. The maximum number of antibodies bound to a CF(1) molecule was three, the angle defined by a neighboring pair of antibodies characteristically being 120 degrees . These results are interpreted as direct evidence for the presence of three alpha subunits in the CF(1) complex, the relative orientation of them being described by 3-fold rotary symmetry. Our observations thus favor an overall subunit stoichiometry of alpha(3)beta(3)gammadeltaepsilon.